1. Field of the Invention
The present invention relates to a molded-article manufacturing method and a mold using gas-assist injection molding, and more particularly, to an optical element including a Fresnel lens.
2. Description of the Related Art
Various injection molding methods have recently been developed to meet the demand to increase the accuracy and function of injection-molded resin articles. One of the injection molding methods is a gas-assist injection molding method that subjects resin to injection molding by injecting gas into a mold during injection molding of the resin. In this method, the gas acts on the resin to decrease the viscosity of the resin, to foam the resin, or to color or reform the resin.
Japanese Patent Laid-Open No. 11-245257 proposes a gas-assist injection molding method that can accurately transfer a surface state of a mold onto an article to be molded. In this method, a cavity of a mold is filled with a specific gas, such as carbon dioxide, at an adequate gas pressure, and fluid resin is also filled therein. Thus, the gas is absorbed by a flow front of the resin, or enters an interface between the mold and the resin and is dissolved in a resin surface layer. The gas dissolved in the resin functions as a plasticizer, and decreases the melt viscosity of the resin, particularly, selectively decreases the setting temperature of the resin surface. When the setting temperature of only the thin resin surface layer thereby falls below the surface temperature of the mold, the resin does not set during the resin filling step. This method enhances the mold surface transferability on an article to be molded.